compression ratio

ok guys here's another question. I'm running 10.1 forged pistons in my motor. From my understanding the actual compression ratio depends on many factors, deck height, head gasket thickness, cylinder volume, etc. Is there anyway to get a more accurate estimate of actual compression ratio by lets say taking a compression reading with a compression gage and using the obtained psi to figure out compression ratio. Else can the psi be used to determine an appropriate cam to use, I do believe the duration on present cam a lunati sp2x may effect psi readings. Inputs please.

 

Allioop108, if you use a gauge you get actual compression with the cylinder's valve leaks and ring blow bye in the actual reading.

The actual compression ratio is calculated by using the cylinder volume and combustion chamber volume (these are calculated using head gasket thickness, deck height, head cc, bore and stroke).

See link
http://www.mtv411.com/auto/Cubic-inch-displacement-liter-calculator.htm

Hope this helps
Mike

 

the guy that did the machine work,on my moter had the books on the piston specs, they go by were the piston sits from top of the deck, also what the type of piston, dished or flat ect... head combustion chamber size. also gasket thickness playes in there. he was able to explain it to me, then i understood. THIS WAS A ONE MAN SHOP THAT ONLY DOES COSTOM WORK AND BALANCEING. i wouldnt talk to a porduction shop, he was able to work with all my varibles to give me what i wanted

 

Cranking VS static compression

As complicated as it seems to calculate compression from bore, stroke, piston dome/dish, deck height, gasket thickness, chamber volume etc. it is harder to estimate true CR from a "compression" test.

The reason is that the valve opening/closing events affect the measured compression when you do a cranking test. So depending on the cam, the same engine can have widely varying cranking compression.

You already know the type pistons in your motor, you should be able to find out the piston dish volume from published specs.

The other three variables that you need to know are the head gasket thickness, piston deck clearance, and combustion chamber volume. Gasket thickness is probably .039 if you are using standard Felpro head gaskets.

Deck height and chamber volume are a guess unless somebody has measured them. You might get close based upon what year your heads are (pre '70 vs. later) and you could calculate for a couple of assumptions, say .050 deck clearance and .010.

Really, the only way you'll know is to pull a head and have the chamber cc'd and measure the deck height. It all depends on how bad you need to know!

 

And when you're done verifying the dimentions, you can grab your cam card and enter the appropriate values in a free download from:

http://cochise.uia.net/pkelley2/DynamicCR.html

read the page, the download is toward the bottom.

Dynamic compression ratio is the static compression ratio as modified by the valve timing. Useful for cam selection purposes.

 

ok then maybe some of you guys with non stock cams (what spec and what compression pistons) can tell me what psi you have when using a compression gage.